Exam 3 Add-on: up to 10 possible bonus points1.
Neurons use electrical and chemical signaling to pass information along specific routes (circuits)
in the CNS and PNS. Within an individual neuron, most information flow is unidirectional,
proceeding from the dendrites to the cell body, then to the initial segment of the axon (trigger
zone), and finally to the synaptic end bulb. Describe the processes, molecule-types, and specific
ions that support this unidirectional flow of information in the neuron. Start with the neuron at
rest and explain what happens when it is stimulated by neurotransmitter at its dendrites.
(Complete in 2-3 paragraphs).
Try to use all of the following terms in your response:
PROCESSES: resting membrane potential; graded potential; depolarization; hyperpolarization;
summation; action potential
MOLECULE TYPES: lipids; leak channels; Na+/K+ ATPase; ligand-gated channels; voltage-gated
channels
IONS: organic anions (inside the neuron); Cl-; Na+; K+
A neuron’s dendrites and cell body can be
bombarded with thousands of depolarizing and
hyperpolarizing graded potentials, all of which
are summed
Action potentials are regenerative
– they do not decay or dissipate
with time and distance from source
Ensures ‘one-way’ communication
down the axon to the axon terminal
Overview of an Action Potential
Resting Phase
• Em = -70 mV
• All voltage-gated
Na+ and K+
channels are
closed
at resting potential
Depolarizing Phase
• Em = -55 mV
• Na+ channels open,
buildup of positive
charges
– Membrane is
depolarized
• AP goes, membrane
potential reaches
+30 mV
Repolarizing Phase
• Em = +30 mV
• Na+ channels
inactivate
• K+ channels open.
• Membrane starts to
repolarize
Repolarizing Phase continues
• Em = -95 mV
• K+ outflow
continues
• Na+ channels
still inactivated
• Membrane
repolarizes
After-hyperpolarizing phase
• Em = -95 mV
• K+ outflow
continues;
channels
eventually close
• Membrane
potential returns
to -70 mV
Refractory Period
Absolute
refractory
period
Relative
refractory
period
16
Action Potentials Propagate from the
Trigger Zone to Axon Terminals
17
Refractory period ensures one-way propagation
18
Action Potentials are All-Or-None
phenomena
19
If APs are All-Or-None, how do they
encode stimulus intensity?
20
Stimulus Strength and AP Frequency Code
Copyright 2010 John Wiley & Sons, Inc.
21
What are the factors that influence
speed of AP conduction?
◼
◼
◼
Temperature
Diameter of axon: larger diameter → faster
conduction
Myalination vs. non-myalination
Copyright 2010 John Wiley & Sons, Inc.
22
Continuous conduction
Unmyelinated fibers
❑
step-by-step
depolarization of
each portion of the
length of the axon
23
Saltatory conduction
Myelinated fibers
❑
Depolarization at nodes
of Ranvier
❑
high density of
voltage-gated Na+
channels
24
Encoding of stimulus intensities
Frequency of action potentials
Sensory neuron activation – more receptors activated leads to more
neurons being activated
25
Synapse – site of interaction between 2 neurons or
between a neuron and an effector cell
Pre-synaptic neuron sends signal
Skeletal
muscle
Post-synaptic neuron receives signal
26
2 Types of synapses
❑
electrical
◼
◼
❑
ionic current spreads to next cell through gap junctions
faster, two-way transmission & capable of synchronizing
groups of neurons
chemical
◼
one-way information transfer from a presynaptic neuron
to a postsynaptic neuron
27
Chemical Synapses
1
Nerve impulse
Presynaptic neuron
2
Ca2+
Ca2+
Voltage-gated Ca 2+
channel
Synaptic end bulb
Cytoplasm
Synaptic
vesicles
Synaptic cleft
Postsynaptic neuron
28
Chemical Synapses
1
Nerve impulse
2
Ca2+
Ca2+
Voltage-gated Ca 2+
channel
Synaptic end bulb
Cytoplasm
Synaptic
vesicles
Synaptic cleft
Ca2+
3
29
Chemical Synapses
1
Nerve impulse
2
Ca2+
Ca2+
Voltage-gated Ca 2+
channel
Synaptic end bulb
Cytoplasm
Synaptic
vesicles
Synaptic cleft
Ca2+
3
Neurotransmitter
receptor
Neurotransmitter
Na+
4
5
Ligand-gated
channel closed
6 Postsynaptic
potential
Ligand-gated
channel open
7
Nerve
impulse
30
Excitatory and Inhibitory Postsynaptic Potentials
◼
The effect of a neurotransmitter can be either excitatory
or inhibitory
❑
a depolarizing postsynaptic potential is called an EPSP
◼
◼
❑
it results from the opening of ligand-gated Na+ channels
the postsynaptic cell is more likely to reach threshold
an inhibitory postsynaptic potential is called an IPSP
◼
◼
◼
it results from the opening of ligand-gated Cl– or K+ channels
it causes the postsynaptic cell to become hyperpolarized
the postsynaptic cell is less likely to reach threshold
31
Summation
32
Removal of Neurotransmitter
◼
Diffusion out of the synaptic cleft
❑
◼
Enzymatic degradation
❑
◼
Acetylcholinesterase (AChE)
Reuptake by neurons
❑
◼
move down concentration gradient
Receptor-mediated endocytosis
Uptake by neuroglia
❑
Neurotransmitter transporters
33
Damage and repair of neurons
❑
CNS
◼
◼
◼
◼
Extremely little to no repair in
the brain and spinal cord
Injury to the brain and spinal
cord is usually permanent
Scar tissue is a barrier to
regeneration
Structure of myelin
sheath/oligodendrocytes
34
Repair within the PNS
◼
Damage to dendrites or axons
35
Ch 12: Introduction to the Nervous System
◼
Sensory function: to sense
changes in the internal and
external environment through
sensory receptors.
❑ Sensory neurons
◼
Integrative function: to analyze
the sensory information, store
some aspects, and make decisions
regarding appropriate behaviors.
❑ Interneurons
◼
Motor function: to respond to
stimuli by initiating action.
❑ Motor neurons
1
Nervous System Divisions: CNS and PNS
◼
Central nervous system (CNS)
❑
consists of the brain and spinal cord
◼
Peripheral nervous system
(PNS)
❑
❑
consists of cranial and
spinal nerves that contain
both sensory and motor
fibers
connects CNS to muscles,
glands & all sensory
receptors
2
Organization of the Nervous System
Copyright 2010 John Wiley & Sons, Inc.
3
Cells of the nervous system
4
Structure of a Motor Neuron
Cell body
Axon
Dendrites
Axon
terminal
Synaptic end bulb
5
Detail of the Cell Body
Initial
segment
Axon hillock
(trigger zone)
Dendrites
Nucleus
Nissl bodies
6
Functional Classification of Neurons
A. Sensory neurons are afferent neurons.
The information is carried from the PNS into the CNS
DENDRITES
AXON
Myelin
sheath
CELL BODY
AXON TERMINAL
7
Some afferent neurons have modified dendrites
Dendrite
(corpuscle of touch)
8
Functional Classification of Neurons
B. Motor neurons are called efferent neurons.
The information is carried away from the CNS
9
Functional Classification of Neurons
C. Interneurons are in the CNS, integrate sensory information
and elicit a motor response
Interneuron
Sensory neuron
Motor neuron
10
Neuroglia of the CNS
Covering
around brain
Microglial
cell
Oligodendrocyte
Myelin sheath
Neuron
Blood
capillary
Astrocytes
Neurons
Ventricle
Ependymal cell
11
Schwann Cells: Neuroglia of the PNS
Myelinated axon
Unmyelinated axons
axons
Schwann cell
Schwann cell
Neurolemma
axon
Myelin sheath
12
Formation of Myelin Sheaths
Nucleus
Cytoplasm
Axon
Myelin sheath
13
Multiple Sclerosis – disease of the CNS
14
Two types of neuronal tissue: gray and white matter
◼
White matter – primarily myelinated axons
◼
Gray matter – neuronal cell bodies, dendrites, unmyelinated
axons, axon terminals, neuroglia.
❑
Nissl bodies cause gray color.
15
Neuron Communication
◼
Neurons are electrically excitable
❑
◼
Changes in voltage alters function
Two basic features allow communication
❑
❑
Presence of ion channels – flow of ions is electrical
current in a biological system
Resting membrane potential – electrical potential
difference (voltage) across the plasma membrane
16
The resting potential is a small build-up of charges
on either side of the neuronal cell membrane
Copyright 2010 John Wiley & Sons, Inc.
17
The resting potential is generated by three features
of neuronal cell biology
Copyright 2010 John Wiley & Sons, Inc.
18
Resting potential exists over the entire surface of
a neuron – input occurs at dendrites/cell body
Cell body
Axon
Dendrites
Axon
terminal
Synaptic end bulb
19
Input is mediated by ligand-gated channels
Copyright 2010 John Wiley & Sons, Inc.
20
Input is mediated by mechanically-gated
channels
Copyright 2010 John Wiley & Sons, Inc.
21
Ion flow through gated channels alters the resting
potential, causing graded potentials
Copyright 2010 John Wiley & Sons, Inc.
22
Ion flow through gated channels alters the resting
potential, causing graded potentials
Copyright 2010 John Wiley & Sons, Inc.
23
Graded Potentials
◼
Hyperpolarizing or
depolarizing
◼
Localized changes in
membrane potential
(opening of channels)
❑
◼
Ligand-gated and
mechanically gated
channels
They can generate an
action potential
24
Stimulus strength
Membrane potential
in millivolts (mV)
Graded potentials vary in amplitude (size)
Resting
membrane
potential
Stimulus 3
Stimulus 2
Stimulus 1
Time in milliseconds (msec)
25
Neuron Communication
◼
Two types of electric signals
❑
❑
graded potentials that are local membrane changes only
action potentials that can travel long distances
graded
potential
action
potential
26
Summation of graded potentials can generate
an action potential (AP)
• 2 stimuli occur close together in time means larger
depolarizing potential.
27
An AP is an all-or-nothing electrical signal
◼
A sequence of rapidly occurring
events
❑ depolarizing phase
❑ repolarizing phase
◼
Voltage-gated Na+ and K+
channels open in sequence
◼
If a stimulus reaches threshold,
the AP is always the same.
❑ A stronger stimulus will not
cause a larger impulse.
28
◼
Action potential
◼
Read the section in your textbook about action
potentials
◼
Visit khanacademy.org and watch the video on
electrotonic potentials and action potentials
❑
Khan academy terminology: electrotonic potential =
graded potential
29
Neuronal Excitability and Communication (Chapter 12)
BIOL212, S. Raft
What is an electrochemical gradient?
For our purposes, we can define ions as charged particles (atoms) that exist in a
solvent (water). Ions can have either a positive or negative charge. Ions in a solvent are free
to move as long as there are no barriers (insulators or other physical obstructions).
Movement or flow of ions in biological systems is like the flow of electrons along a copper
wire in an electrical device: both are examples of current.
Unobstructed, ions of a particular type – say sodium ions (Na+) – will generally move
from a region of high Na+ concentration to a region of low Na+ concentration, and will
continue to do so until the Na+ concentration is uniform throughout the region of interest; this
is called movement down a concentration or chemical gradient (let’s stick with the term
chemical gradient here).
Unobstructed, ions will also move in such a way as to maximize their distance from
ions of like charge and minimize their distance from ions of opposite charge (likes repel;
opposites attract); this will continue until the solution is net neutral in charge. This is called
movement down an electrical gradient.
When many types of ions co-exist in a single system (as is always the case in biology)
we must put these two concepts together. We say that – without input of energy into a
system – ions always move down their electrochemical gradient. This involves net
movement of a particular ion until its chemical and electrical gradients are at equilibrium,
when the force of a chemical gradient is exactly balanced by the force of an electrical
gradient. Unusual things can happen when chemical and electrical gradients interact before
reaching equilibrium. For example, if a concentration gradient of potassium ions (K+) is very
steep and an electrical gradient (in the opposite direction) is very small, we might see net
movement of K+ ions toward a region of positive charge! In this case, the concentration
gradient overwhelms the force of the electrical gradient, and the movement of K+ toward the
more positive region will continue until the two forces (electrical and chemical) balance one
another exactly.
What is a resting potential?
Both extracellular (interstitial) and intracellular (cytosol) fluids contain a complex
mixture of ions and are net neutral in charge (see Fig. 12.11a and 12.12 in your textbook).
Furthermore, extracellular and intracellular fluids always have different compositions of ions.
This is because: 1) cells have many active (energy-dependent) mechanisms for controlling
the types of ions present in the cytoplasm; and 2) the cell membrane (lipid bilayer) is a barrier
to movement of ions (it is a good electrical insulator). But the cell membrane is not a perfect
insulator and ions of opposite charge can attract one another across the lipid bilayer even
though their movement is obstructed by it. This results in a small buildup of opposite charges
on either side of the cell membrane. In neurons at rest (not stimulated by input) this
small buildup of opposite charges on either side of the cell membrane is called the
resting potential. It is called the resting potential because a small voltage is established
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Neuronal Excitability and Communication (Chapter 12)
BIOL212, S. Raft
across the cell membrane and voltage is a form of potential energy. In most neurons the
resting potential is around -70 mV (millivolts); negative voltage indicates that the charge at
the inner (cytoplasmic) side of the membrane is negative relative to the outer (extracellular)
side of the membrane. We can think of the resting potential as a ground state on which the
code for neural communication is built.
How is the resting potential generated?
Neuron cytoplasm has high concentrations of potassium (K+) cations (positive ions)
and organic anions (negative ions). By ‘organic’, we mean that much of the negative charge
in the cytosol is due to negative charges on phosphate groups (e.g., on ATP) and amino acid
side chains. There are comparatively low concentrations of K+ and organic anions in the
extracellular fluid.
By contrast, extracellular fluid in the nervous system has high concentrations of Na+
cations and chloride (Cl-) anions. There are comparatively low concentrations of Na+ cations
and Cl- anions in the neuron cytoplasm
The origin of differences in ionic concentrations between neuronal cytoplasm and
extracellular fluid is a complex topic beyond the scope of this course. However you are
required to understand that these concentration differences are a pre-condition for generation
of the neuron resting potential (as well as for generation of the graded potential and action
potential).
What you do need to know is that, given these concentration differences, three
features of neuronal cell biology explain the generation of the resting potential (you
should refer to Fig. 12.12 in your textbook):
First, the neuronal cell membrane has so-called leak channels: these are cationselective protein pores that allow either Na+ or K+ to cross from one side of the membrane to
the other. Leak channels are open all the time, and the neuron has many more K+ leak
channels than Na+ leak channels. This means that at rest, the neuronal membrane is far
more permeable to K+ than to Na+. Since K+ is able to leak out of the neuron (down its
concentration gradient) faster than Na+ can leak in (down its concentration gradient), some
extra positive charge is deposited on the outer surface of the neuronal membrane.
Second, the neuronal cell membrane is completely impermeable to organic anions,
the major class of negative charges in the neuronal cytoplasm. This means that a negative
charge can’t ‘follow’ the leak of K+ out of the neuron. Again, this contributes to the inside of
the membrane being more negative and outside being more positive.
Third, the neuron has many sodium-potassium pumps in its cell membrane. The
sodium-potassium pump actively (in an energy-dependent manner) and continuously exports
three Na+ ions from cytoplasm to extracellular fluid while importing two K+ ions from the
extracellular fluid to the neuronal cytoplasm. This unequal transfer of ions causes a net
2
Neuronal Excitability and Communication (Chapter 12)
BIOL212, S. Raft
charge of +1 to be sent out of the neuron each time the pump cleaves one ATP molecule
(and it cleaves a lot of ATP).
You must understand these three features as causing the resting potential.
What causes a graded potential?
All neurons receive input, either from other neurons or from the external/internal
environment. External/internal sensory input is received at the dendrites or modified
dendrites of a sensory neuron. Input from other neurons (to interneurons or motor neurons)
is received at the dendrites or the neuronal cell body. Input may take the form of physical
deformation (e.g., stretch, pressure, etc) or a chemical signal (e.g., a neurotransmitter
released by another neuron). At the dendrites or cell body, physical deformation is mediated
by mechanically-gated channels, while chemical signals are mediated by ligand-gated
channels.
Gating (opening) of either channel type in response to stimuli results in a selective
(Na+ or K+ or Cl-) flow of ions through the channel and down the ion’s electrochemical
gradient. In other words, there is a localized current across the neuronal cell membrane and
a spatially localized alteration of the resting membrane potential. This spatially localized
alteration of the resting membrane potential at the dendrites or cell body and in
response to a stimulus causes graded potentials. Therefore, graded potentials are the
immediate response of neurons to a stimulus.
What are the characteristics of graded potentials?
Up to this point, we have learned that opening of mechanically- or ligand-gated
channels at the dendrites or cell body in response to stimuli causes a very brief and localized
flow of current across the neuronal cell membrane. Current flow relies on differential
concentrations of ions in the intracellular and extracellular fluids. The current flow has a
transient and localized effect on the separation of charge at either side of the neuronal cell
membrane; in other words, current flow influences the resting potential (see Fig. 12.13).
If the gating of channels allows a selective flow of Na+ ions, positive charge will flow
into the cell, because Na+ will flow down its chemical gradient (remember, more Na+ outside)
and down its electrical gradient (Na+ will flow toward the buildup of negative charge on the
inside of the resting cell membrane). This will locally (where the channels opened) alter the
resting potential by making the inner surface of the cell membrane more positive relative to
the outer surface (see Fig. 12.15b in your textbook). We call this a depolarizing graded
potential, because it makes the voltage difference across the cell membrane less negative
and therefore smaller (closer to zero voltage). We also call this an excitatory postsynaptic
potential (EPSP) because (as you will soon see) the ‘firing’ of an action potential by a neuron
in response to stimuli occurs when the resting potential at a trigger zone (initial segment of
the axon) surpasses a threshold of about -55mV (more on this below).
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Neuronal Excitability and Communication (Chapter 12)
BIOL212, S. Raft
If the gating of channels allows a selective flow of K+ ions, positive charge will flow out
of the cell, because K+ will flow down its chemical gradient (remember, more K+ inside). This
chemical gradient is so strong that it causes K+ ions to flow against its electrical gradient (a
positive ion flows toward the positively charge outer surface of the membrane), but as I stated
above, unusual things can happen in cellular electrophysiology. Outflow of K+ ions locally
alters the resting potential by making the inner surface of the membrane more negative
relative to the outer surface. We call this a hyperpolarizing graded potential, because it
makes the voltage difference across the cell membrane more negative and therefore larger.
We also call this an inhibitory postsynaptic potential (IPSP) because it can drive the
resting potential at the trigger zone further away from threshold, thereby inhibiting the neuron
from firing an action potential.
All graded potentials, regardless of whether they are depolarizing or hyperpolarizing,
share certain characteristics. First, graded potential vary in strength (hence the name
graded) according to the strength of the mechanical or chemical stimulus (see Fig. 12.14 in
your textbook). Second, their effects on the resting membrane potential are local. As graded
potentials spread along the membrane from a point of origin, their strength and effect on the
resting membrane potential decreases. In short, graded potentials decay or dissipate over
time and space. How then, can graded potential have any effect on a neuron’s firing of an
action potential? How does the a transient graded potential that begins at a dendrite work it’s
way to the trigger zone of the axon at the other end of the neuron cell body?
Graded potentials can influence neuronal firing of action potentials because most
neurons are simultaneously stimulated at hundreds or even thousands of synaptic inputs.
The many resulting graded potentials summate to provide a ‘weighted average’ of all the
inputs to the neuron (see Fig. 12.16 in your textbook). The summated activity of so many
inputs to the dendrites and cell body of the neuron can sufficiently alter the resting potential at
the trigger zone so that the neuron either suppresses or fires an action potential.
More stuff on resting and graded potential:
At this point, I strongly recommend that you watch the following two Khan Academy
videos:
‘Neuron Resting Potential Description’
‘Neuron Graded Potential Description’
Search for them by name on the Khan Academy home page.
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Neuronal Excitability and Communication (Chapter 12)
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Where are we going next?
Everything we’ve covered up to now has to do with the physical basis of neuronal
excitability and the manner in which the neuron processes inputs to its dendrites and cell
body. As mentioned above, the summated input affects resting potential at the initial
segment of the axon (trigger zone), and this determines whether the stimulated neuron will
pass the ‘message’ along to other neurons. It can do this by firing an action potential. Action
potentials differ from graded potentials in a number of ways (see Table 12.1 in your
textbook).
Next time we meet, I will describe the mechanism of the action potential. Before then,
I strongly recommend that you READ THE SECTION ON ACTION POTENTIALS IN YOUR
TEXTBOOK and watch the following Khan Academy video:
https://www.khanacademy.org/science/biology/human-biology/neuron-nervoussystem/v/electrotonic-action-potential
This video compares and contrasts the graded potential and the action potential (note: here,
graded potential is referred to as electrotonic potential – these are just two names for
the same thing).
5
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